A grate for use in refuse combustion furnaces is customarily formed by rows of grate bars lying one behind the other and extending transverse to the direction of transport of the material being burned. These grate bars are subjected to considerably higher thermal and mechanical loads than grate bars used in power plants fueled by fossil fuel, since the materials burned in these combustion furnaces, vary considerably, in size, weight, calorific power, burning behavior and the like. The life of a grate bar should be as long and constant as possible for reasons of repair and overall availability of the furnace.
In a refuse combustion furnace, there are a myriad of different operational requirements, which are attempted to be satisfied, by varying the shape of the grate bars, and by utilizing grate bars of different materials.
A hollow box-shaped bipartite grate bar having air fed conduits on its rear and outlet openings on its surface is described in European Patent B1-0 205 658. Grate bars of this type produce a constant minimum air pressure in order to prevent the material being burned from penetrating into the grate bars. To date, however, no attention has been paid to the problem caused by melting non-ferrous metals and small high density steel scraps.
A grate bar in accordance with European Patent A1 0 170 803 comprises a support part and a head part which is flanged to the front of the grate bar and which is made of a material having great thermal resistance. During oxidation of the materials being burned, the grate bars receive part of the air from the bottom of the grate and, expel the air from the head part of each grate bar through an air outlet opening. Relative to the bottom of the grate there is a vacuum within the combustion chamber of the furance. The grate bar, which is otherwise open at the bottom, is divided by a rib at the center for reasons of stability and conduction of air. However, the above-described grate bar has certain defects. Molten, non-ferrous metals and small parts of steel scrap can be trapped in the space between the support part and the head part of the grate bar. Because the grate bar supports are disposed on the bottom of the grate bars, and because of the special shape of the head part of the grate bars, the movable rows of grate bars cannot, under these circumstances, be fully moved over the top surfaces of the adjacent rows of grate bars. Moreover, the air cooling of the bars is not constant due to variable inflow cross sections, as a result of the movement of the grate bars.
German Patent A1-33 13 615, discloses grate bars which have, at their head part, an outlet opening which permits the emergence of a stream of air at an angle which extends beyond the horizontal plane of the grate as well as beyond the head of the adjacent row of grate bars. The grate bar is otherwise open at the bottom and therefore does not permit a constant air cooling of the grate bars.
The top surfaces of the grate bars are arranged in a step-wise manner and the stroke of each row of grate bars is limited to about one-half of the free top surface of the grate bars.
German Patent A1 38 13 441, discloses a grate bar which is configured as a double layer construction for admitting air in two planes, and consequently, with two planes permitting the air to exit at the protruding head part of the grate bar. The front surface of the lower grate bar above the air outlet plane of the upper grate bar can never be cleared from any material including material to be burned, non-ferrous metals and steel scrap. Thus, such grate bars can only be used effectively on oblique grates and not on horizontal grates. No consideration has been given to the problem of melting non-ferrous metal.